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Arduino While loop

The Arduino while loop is another loop control structure that lets you conditionally repeat a block of code. It is different from the for loop discussed in the previous part of this programming course in that it does not have the initialiser or incrementer parts - you set these up outside the while loop.

The While loop

The while keyword uses the expression (within the parentheses following the the while keyword) as a control. If the expression is true (not zero) then the block of code that follows the expression is repeated.

while ( <conditional expression> )
{
   < Block of code to execute>
}

The only way to exit the loop is if the conditional expression changes - that means you have to use a variable or input pin value that allows the expression to become zero at some point. Otherwise you stay in the loop!

The do while loop

There is an alternative form of the while loop called the do-while loop and this is different to the while loop, only in the fact that it always executes the block of code at least once regardless of the conditional expression.

do
{
   < Block of code to execute>
} while ( <conditional expression> )

This is often useful when you must perform some code but then want to test to see if you should carry on repeating it.

Arduino While loop examples

While loop example 0 to 9

In the for loop tutorial a variable was incremented 10 times and the value printed out. You can do the same operation using the while loop.

Here's the code from the for loop example

for (int i=0; i<10; i++) Serial.println(i);

Here's the new sketch for using the while keyword:

void setup (void) {
int i=0;

   Serial.begin(9600);

   Serial.println("Arduino while loop");

   while(i<10) {
      Serial.println(i);
      i++;
   }
}

void loop(void) {
}

Here is the output from the sketch

Arduino while loop
0
1
2
3
4
5
6
7
8
9

You can see that the same operation is acheived and you can also see that there are the same elements that are used in the for loop:

Parameter	Example	Difference to for loop code.
Initialiser:	int i=0;	Placed before the while keyword.
Condition:	i<10;	Placed in parenthesis after the while keyword.
Iterator:	i++;	Placed within the body code.

Arduino While loop example 1 through 10

By moving the iterator (i++) you can change the output as a squence from 1 to 10 - this is easier than the for loop logic as you don't need to think of the conditional i.e. should it be >=10, <11 etc. In fact the for loop for iterating 1 through 10 would be:

for (int i=1; i<=10; i++) Serial.println(i);

But you have to think about it!

Heres the while loop Sketch for values 1 through 10:

void setup (void) {
int i=0;

   Serial.begin(9600);

   Serial.println("Arduino while loop");

   while(i<10) {
      i++;
      Serial.println(i);
   }
}

void loop(void) {
}

The only change is to move the line i++ before the body code (here Serial.println).

While loop 1-10 output

Arduino while loop
1
2
3
4
5
6
7
8
9
10

Arduino example of the Do while loop

Here the condition is tested at the end so the main body of code is always executed once.

void setup (void) {
int i=0;

   Serial.begin(9600);

   Serial.println("Arduino do while loop");

   do {
      i++;
      Serial.println(i);
   } while(i<10);
}

void loop(void) {
}

Here's the output of the above code

Arduino do while loop
1
2
3
4
5
6
7
8
9
10

Difference between for loop and a while loop

The difference between the for loop and a while loop is that the for loop uses a specific variable in the initialiser and no other. The while loop can use any variable that has been defined before the while keyword.

The other difference is that using the do-while loop makes it easy to perform the action always once (regardless of the conditional expression) and you can not do that with the for loop.

You will come across situations where you want to perform the action always once, so remember to use the do while loop instead of trying to write out complicated flag conditions using other variables.

Test Yourself

Q: There is no difference between while and for loops (T/F) ?

True or false (click to find out)

False: You can use a do while loop to execute a block always once which is not possible with a for loop.

Q: While loops and for loops can never perform the same way (T/F)?

True or false (click to find out)

False: You can make a while loop do the same job as a for loop.

The infinite while loop

The one unusual thing you can do with the while construct is to create a never ending loop. This may sound ridiculous but processors don't know how to find the start and end of memory.

The processor just increments the program counter (this just points to code stored in flash memory). If there is no instruction not to stop (or jump somewhere else) it just keeps going until it runs out of memory.

In Arduino code, the loop() construct contains an infinite while loop.

An infinite while loop is just a while loop with the conditional set to true. As with the normal loop code begins again when the program reaches the last closing brace and jumps to the start brace. The difference here is that the conditional is always true so the program counter never exits from the loop.

Of course the reset button (or power on/ off) is the exit mechanism!

void setup (void) {

   while(1) {
      // Do something forever
   }
}

void loop(void) { //This is now redundant!!!
}

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